This invention relates to a semiconductor laser and an optical pickup with the semiconductor laser. Particularly, this invention relates to an optical pickup with a semiconductor laser which are suitable for reading information recorded on optical information storage media.
In optical discs such as Compact Disc (CD) and Digital Versatile Disc (DVD), and recordable discs (photo-electro-magnetic discs) such as MiniDisc (MD) and Magnetic-Optical disc (MO), a laser beam output from a semiconductor laser is irradiated on an information recording layer of a disc, and light reflected therefrom is detected to read the content of the recorded information.
FIG. 1 shows a general state of above, in which a laser beam output from a semiconductor laser 10 is incident onto a beam splitter 12. The laser beam having transmitted through the beam splitter 12 is focussed on an information recording layer of a disc 16 by an objective lens 14. A reflected laser beam subjected to modulation according to the information recorded on the disc 16 transmits through the objective lens 14 and is incident onto the beam splitter 12. The laser beam reflected by the beam splitter 12 is incident onto a photo detector 18 where the laser beam is detected as an electric signal. For the beam splitter 12, a half mirror or a polarization beam splitter are used.
In such an optical disc device as described, out of the outgoing bundle of laser beams from the semiconductor laser 10, a part reflected by the information recording layer of the disc 16 transmits through the beam splitter 12 and is fedback to the semiconductor laser 10. Phenomena such as a longitudinal mode competition or a mode hop occur due to the return beam to appear as a noise so called an intensity noise, which is generally known as "backtalk noise".
The backtalk noise brings forth a lowering of quality of a read signal and an unstable operation. It is therefore necessary for performing a better operation to reduce or remove the backtalk noise. Particularly, in video discs in the LD format, photo-electro-magnetic discs such as MD, MO or the like, and DVD which has low (narrow) system tolerance due to high density, it is contemplated that an improvement against the backtalk noise is essential, so that various countermeasures have been taken. General countermeasures are listed as follows which are methods put to practical use in an optical disc player or the like.
(1) .lambda./4 plate (.lambda.: wavelength) and polarization beam splitter (PBS) PA1 (2) Beam splitter-branch ratio adjustment PA1 (3) High frequency modulation (superposition) and PA1 (4) Self-oscillation (Self-pulsation)
The countermeasures (1) and (2) are to reduce the backtalk intensity itself. On the other hand, the countermeasures (3) and (4) are to increase a line width of oscillation spectrum, in other words, to lower time coherence to thereby relatively reduce a noise amplitude.
The countermeasures listed above, however, have following inconveniences.
(1) .lambda./4 plate and PBS: There is a rise in cost of a price of .lambda./4 plate, and a difference in price between a polarization beam splitter and a normal beam splitter. In addition, there are inconveniences of complication of assembly and larger-size due to an increase in number of parts, and restriction to miniaturization of the apparatus itself.
(2) Beam splitter-branch ratio adjustment: A satisfactory effect is not obtained, a rise in cost rarely occurs though.
(3) High frequency modulation (superposition): A high frequency superposition circuit (unit) has to be added, resulting in a rise in cost. Further, the high frequency superposition circuit is required to be directly mounted on an optical pickup casing, leading to a larger-size of a movable part and an increase in mass, and to deterioration of access performance. Moreover, there occurs an interference and disturbance to other circuits due to a leakage of an electromagnetic wave, and an electromagnetic shield is necessary to suppress it, rising a cost. In addition, at the time of high frequency superposition, laser will be an intermittent emission in principle. For this reason, even in the same average optical power, an instantaneous peak value of optical intensity is approximately twice that of continuous oscillation, resulting in a laser facet (end) deterioration and a deterioration of reliability which depends on the instantaneous peak value.
(4) Self-oscillation: It is difficult to set conditions of a design and a process for obtaining an operation of self-oscillation having a sufficient strength. Further, at present, it is difficult to keep balance between the self-oscillation and the reduction in threshold current desired in terms of system.